This invention relates generally to air filters, and more specifically to a non-woven filter composite and a method for forming the composite.
The removal of air borne contaminants from the air is a concern to everyone. Gas phase filtration has traditionally been accomplished by methods which utilize activated carbon. One approach has been to use a carbon/adhesive slurry to glue the carbon to a substrate. However, the adhesive decreases carbon performance by forming a film on its surface. A second approach involves carbonizing an organic based web by heating, followed by carbon activation. This material has a high cost and has relatively low adsorption capacity. A third approach involves forming a slurry of carbon powders and fibers into sheets by a process analogous to a wet papermaking process. This material is medium-to-high cost, and has an undesirable high pressure drop.
Alternatively, carbon particles have been treated with chemicals to increase uptake of air contaminants. However, chemical treatment is not efficient when used in conjunction with an aqueous process, as the aqueous nature of the process either washes away the chemical used to impregnate the carbon, or reacts undesirably with the impregnating chemical rendering it useless. However, filter materials which do not incorporate chemical absorbents into the carbon particles perform far less effectively than those which do include chemically impregnated absorbents.
Another approach to entrain air contamination has been to produce low, medium and high efficiency pleatable composite filter media which include either a low, medium or high efficiency fibrous filtration layer of randomly oriented fibers; and one or more permeable stiffening layers which enable the composite filter media to be pleated and to sustain its shape. Such filtration devices serve as vehicle passenger compartment air filters, high performance engine air filters and engine oil filters. ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) pleatable filters and the like typically use a pleated high efficiency filtration media for the filtration element.
Currently, the pleated high efficiency media normally used in these filtration devices are made from ASHRAE filter media or paper products. These paper products are made by a wet-laid technique wherein fibers, e.g. glass or cellulosic fibers, are dispersed in an aqueous binder slurry which is stirred to cause the fibers to become thoroughly and randomly mixed with each other. The fibers are then deposited from the aqueous binder slurry onto a conventional paper making screen or wire as in a Fourdrinier machine or a Rotoformer machine to form a matted paper which includes a binder resin, e.g., a phenolic resin. Pleated filter elements made from such papers can exhibit high efficiencies. However, these pleated filter elements have low dirt-holding capacities and exhibit high pressure drops.
Electrostatically charged synthetic filter media is also used in these filtering applications, and these can attain very high filtration versus pressure drop performance characteristics, at least in their initial charge state. However, during use many of these products lose their electrostatic charge, or it is masked by deposits, causing filtration efficiency to drop substantially, sometimes to levels below what is acceptable.
Accordingly, there remains a need to provide a relatively low cost, high efficiency filter media for these filtration applications which exhibit relatively high dirt-holding and/or air contaminant capacities and relatively low pressure drops as well as low and medium efficiency filter media which exhibit relatively high dirt-holding capacities and relatively low pressure drops.
The present invention circumvents the problems described above by providing fiber webs and filter composites which retain particles, air borne contaminants, and/or oil without reduction in filtration performance below a high base threshold even after prolonged filtration challenges. In a particular embodiment, the filter media of the present invention is a polymeric fiber web having a fine fiber layer on a coarser support, and which, after decay of any charge which may be present, possesses an alpha above about 11, i.e. 13 or 14. The web can include an antioxidant within the web matrix. Accordingly, the present invention provides filter media, useful in filtering applications such as air conditioning, ventilation and exhaust ducts as bag filters or pleated panel filters, which relies upon mechanical filtration properties rather than electrostatic charge for its base level of filtration efficiency, thus providing filter media which have enhanced filtration performance characteristics, such as efficiency versus pressure drop characteristics over time.
The present invention comprises a cost effective, high efficiency, low pressure drop, adsorptive, non-woven filter media comprising a high surface area synthetic microfiber, e.g., melt blown, fine fiber layer. The filter media can also include one or more non-woven spun bond layers and can be combined with a coarse fiber support layer. The coarse fiber support layer can itself be a low pressure drop synthetic microfiber, e.g., melt blown, layer adhered to a spun bond layer, and can serve as a prefilter to enhance overall performance. The invention also contemplates a method for forming the filter media comprising dry application of the non-woven fine fiber filter media to the non-woven carrier material. Various layers can be calendared, and the complete multilayer web assembled with heat, with or without a cover sheet.
In one exemplary method of manufacture, a polypropylene resin is extruded by a melt pump through a die having a plurality of extrusion holes to produce large diameter fibers into a stream of hot air which stretches the fibers to a diameter well below several microns and carries them toward a collector belt passing over a vacuum box opposite the extrusion head. Preferably a spun bond mat or web is carried by the collector belt so that the melt-blown fine fibers land on and accumulate on the spun bond layer to produce a large area matrix of filtration material on a support. This material can be used directly on a suitable structural support, or can be further bonded to another layer of course non-woven fibrous support material to form a large area filter media suitable for a variety of commercial uses. Alternatively, the synthetic microfiber fine fibers can be used alone, and the web carried on the collector belt is used only to collect the blown fibers. The synthetic microfiber fine fibers of the invention have an alpha value of at least about 11 or more, i.e., 13 or 14. In a particularly preferred embodiment, the synthetic microfiber fine fibers have an alpha value which remains constant or stable over time. The synthetic microfiber fine fiber web can be calendered to enhance fiber entanglement. In a preferred embodiment, the synthetic microfiber is a melt blown fiber.
In one embodiment, the present invention pertains to filter media which include an effective filtration layer of synthetic microfiber which need not be charged. The actual diameter of the fibers of the synthetic microfiber material is between about 0.8 to about 1.5 microns, i.e. 1.0 microns, as measured by scanning electron microscopy, and in a preferred embodiment, the synthetic microfiber, e.g., melt blown, polymeric material is a polypropylene, e.g., Exxon PP3456G (Exxon, Houston, Tex.) having a melt flow of about 1200, which contains an antioxidant. Preferably the fine fiber has a web basis weight of between about 6 g/m2 and about 25 g/m2, and is generally applied over a coarser support or strengthening layer of low solids such that the web has an alpha value of at least about 11 or more, i.e., 13 or 14. The synthetic microfiber fine fibers of the invention have a 60-65% ASHRAE (at a basis weight of about 6 to about 12 g/m2, e.g., 8 g/m2), 80-85% ASHRAE (at a basis weight of about 15 g/m2 to about 22 g/m2, e.g., 18 g/m2) and 90-95% (at a range of about 18 g/m2 to about 25 g/m2). In a particularly preferred embodiment, the polymer fiber web has an alpha value which remains constant or stable over time.
In another embodiment, the present invention pertains to filter media which include a synthetic microfiber, e.g., melt blown, polymer fine fiber layer which is substantially uncharged, and at least one spun bond fiber or coarse fiber support layer. The diameter of the synthetic microfiber fine fibers is between about 0.8 to about 1.5 microns, i.e. 1.0 microns, and the spun bond or coarse fiber layer acts as a support, prefiltering or strengthening layer. As applied to the upstream side of the filter in a vent or air conditioning flow, the spun bond fiber layer can have a basis weight of between about 5 g/m2 and 10 g/m2, e.g., 8.5 g/m2, and serves as a prefilter. As applied to the downstream side, it can be applied in a layer two to four times more massive, enhancing its function as a support web. The stiffer backing can have a basis weight between about 34 g/m2 and about 55 g/m2, i.e., 40.8 g/m2 and about 54.4 g/m2). Typically the spun bond material is selected from polyesters, polyethylene, polypropylene, or polyamide polymers, and is assembled with the fine fiber layer such that the filter media composite has an alpha value of about 11 or more, i.e., 13 or 14. In a preferred embodiment, the spun bond layer is made of a polypropylene resin manufactured by Reemay. For example, a preferred polypropylene spun bond support with a basis of weight of 40.8 g/m2 is fabricated from TYPAR 3121N (Reemay, Old Hickory, Tenn.) and for a polypropylene spun bond support with a basis weight of about 54.4 g/m2 is fabricated from TYPAR 3151C (Reemay, Old Hickory, Tenn.).
Alternatively, a coarse synthetic microfiber, e.g., melt blown, material which serves as a prefilter can be used as a support and typically has a basis weight between about 50 g/m2 to about 100 g/m2, e.g., 80 g/m2, with a fiber diameter of between about 5 and about 20 microns, e.g., between about 13 and about 17 microns, e.g., between about 13 and 15 microns. For example, the coarse melt blown material can be made from a polypropylene resin having a melt flow of 1200 (polypropylene resin PP3546G, Exxon, Houston, Tex.) or, preferably, a polypropylene having a melt flow of 400 (polypropylene resin HH441, Montell Polymers, Wilmington, Del.). The coarse synthetic microfiber is assembled with the fine fiber layer such that the filter media composite has an alpha value of at least about 11 or more, i.e., 13 or 14.
The combination of the synthetic microfiber, e.g., melt blown, fine fiber with the spun bond fiber layer or coarse synthetic microfiber, e.g., melt blown, layer in a web is unique in that no bonding agents, e.g., adhesives, are required to adhere the two materials to each other. Typically, the two layers are pressed together by a calendering process which causes each layer to physically adhere to the other layer. This provides the advantage that a bonding agent is not incorporated into the composite and does not effect the porosity of the composite filter media.
In still another embodiment, the present invention pertains to filter media which includes a substantially uncharged synthetic microfiber layer that can provide an effective degree of filtration when charge, if any, is dissipated, a spun bond fiber layer and a coarse support fiber layer. Generally, the actual diameter of the synthetic microfiber fine fibers is between about 0.8 to about 1.5 microns and preferably about one micron, and the coarser support fiber layer acts as a support for the finely enmeshed fine fiber web material. Typically the coarse support fiber layer is made of polymers which can also be blown but have lower solids and can, for example have a much higher stiffness and greater fiber diameter. The coarse synthetic microfiber material which serves as a prefilter has a basis weight between about 20 g/m2 to about 100 g/m2, e.g., 80 g/m2, with a fiber diameter of between about 5 and about 20 microns, e.g., between about 13 and about 17 microns, e.g., between about 13 and 15 microns. For example, the coarse synthetic microfiber material can be made from a polypropylene resin having a melt flow of between about 400 and about 1200. For example, a suitable polypropylene resin with a melt flow of 1200 is the polypropylene resin PP3546G, available from Exxon (Houston, Tex.). Preferably, a polypropylene resin having a melt flow of 400 is available from Montell Polymers (Wilmington, Del.), designated as HH441. Preferably, the fine fiber layer is applied to the coarser layers to produce a filter media composite having an alpha value of at least about 11, i.e. 12, 13 or 14. In a preferred embodiment, the synthetic microfiber is a melt blown fiber.
The present invention also pertains to filter media which include a first spun bond fiber layer, a substantially uncharged synthetic microfiber layer, a coarse support fiber layer and a spun bond support fiber layer. It should be understood that additional layers of each material can be included to form the final composite filter media web for particular applications or strength requirements. It should also be understood that the order of layers can be switched so long as the layers are assembled so that an alpha of 11 or more, i.e., 13 or 14, is achieved in the uncharged or charge-decayed state, and typically, the synthetic microfiber fine fiber filter media has fibers of a diameter of between about 0.8 to about 1.5 microns. In general, the filter media of the present invention can be fabricated in a range, e.g., with particle penetrations of 60-65 percent ASHRAE, 80-85 percent ASHRAE, or 90-95 percent ASHRAE, while still achieving the combination of effective filtration for the level of pressure drop. In representative examples below, the amounts of fine fiber can be varied for the different embodiments.
All percentages by weight identified herein are based on the total weight of the web unless otherwise indicated.